Frequency modulated microphone system

ABSTRACT

Systems and methods of sensing audio with a MEMS microphone that modulates a frequency of a phase-locked loop. The MEMS microphone includes a movable electrode and a stationary electrode. The movable electrode is configured such that acoustic pressures acting on the movable electrode cause movement of the movable electrode. A voltage-controlled oscillator of the phase-locked loop is coupled to the MEMS microphone and receives a control signal. The voltage-controlled oscillator also generates an oscillating signal based on the control signal and a capacitance between the movable electrode and the stationary electrode. A phase detector of the phase-locked loop receives and determines a phase difference between the oscillating signal and a reference signal. The phase detector further generates the control signal based on the phase difference. A controller is configured to receive the control signal and determine an audio signal based on the control signal.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/987,198, filed on May 1, 2014 and titled “FREQUENCY MODULATEDMICROPHONE SYSTEM,” the entire contents of which is incorporated byreference.

BACKGROUND

Embodiments of the invention relate to systems and methods of sensingaudio with a microphone using frequency modulation. More specifically,the embodiments of the invention relate to audio sensing with amicro-electro-mechanical system (MEMS) microphone.

SUMMARY

In some implementations, one embodiment of the invention provides, amongother things, a microphone system. The microphone system includes a MEMSmicrophone, a phase-locked loop, and a controller. The MEMS microphoneincludes a movable electrode and a stationary electrode. The movableelectrode has a first side and a second side that is opposite the firstside. The movable electrode is configured such that acoustic pressuresacting on the first side and the second side of the movable electrodecause movement of the movable electrode. The stationary electrode ispositioned on the first side of the movable electrode. The phase-lockedloop includes a voltage-controlled oscillator and a phase detector. Thevoltage-controlled oscillator is coupled to the MEMS microphone. Thevoltage-controlled oscillator receives a control signal. Thevoltage-controlled oscillator generates an oscillating signal based onthe control signal and a capacitance between the movable electrode andthe stationary electrode. The phase detector is coupled to thevoltage-controlled oscillator. The phase detector receives theoscillating signal and a reference signal. The phase detector detects aphase difference between the oscillating signal and the referencesignal. The phase detector also generates the control signal based onthe phase difference. The controller is coupled to the phase-lockedloop. The controller is configured to receive the control signal anddetermine an audio signal based on the control signal.

In other implementations, the invention provides a method of sensingaudio with a MEMS microphone. The MEMS microphone includes a movableelectrode and a stationary electrode. The movable electrode has a firstside and a second side that is opposite the first side. The movableelectrode is configured such that acoustic pressures acting on the firstside and the second side of the movable electrode cause movement of themovable electrode. The stationary electrode is positioned on the firstside of the movable electrode. The method includes receiving, with avoltage-controlled oscillator, a control signal. The method alsoincludes generating, with the voltage-controlled oscillator, anoscillating signal based on the control signal and a capacitance betweenthe movable electrode and the stationary electrode. The method furtherincludes receiving, with a phase detector, the oscillating signal and areference signal. The method also includes detecting, with the phasedetector, a phase difference between the oscillating signal and thereference signal. The method further includes generating, with the phasedetector, the control signal based on the phase difference. The methodalso includes receiving, with a controller, the control signal. Themethod further includes determining, with the controller, an audiosignal based on the control signal.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of a MEMS microphone.

FIG. 2 is a block diagram of a phase-locked loop.

FIG. 3 is a block diagram of a phase detector in the phase-locked loopof FIG. 2.

FIG. 4 is a block diagram of a ring oscillator.

FIG. 5 is a block diagram of a CMOS inverter in the ring oscillator ofFIG. 4.

FIG. 6 is a block diagram of a microphone system.

FIG. 7 is a block diagram of the MEMS microphone of FIG. 1 and the CMOSinverter of FIG. 5.

FIG. 8 is a block diagram of the MEMS microphone of FIG. 1 and the CMOSinverter of FIG. 5.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

Also, it is to be understood that the phraseology and terminology usedherein is for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising” or “having” andvariations thereof herein is meant to encompass the items listedthereafter and equivalents thereof as well as additional items. Theterms “mounted,” “connected” and “coupled” are used broadly andencompass both direct and indirect mounting, connecting and coupling.Further, “connected” and “coupled” are not restricted to physical ormechanical connections or couplings, and can include electricalconnections or couplings, whether direct or indirect. Also, electroniccommunications and notifications may be performed using other knownmeans including direct connections, wireless connections, etc.

It should also be noted that a plurality of hardware and software baseddevices, as well as a plurality of different structural components maybe utilized to implement the invention. Furthermore, and as described insubsequent paragraphs, the specific configurations illustrated in thedrawings are intended to exemplify embodiments of the invention.Alternative configurations are possible.

In some implementations, a MEMS microphone 100 includes, among othercomponents, a movable electrode 105 (e.g., membrane) having a first side107 and a second side 108, a stationary electrode 110, and a barrier115, as illustrated in FIG. 1. The stationary electrode 110 ispositioned on the first side 107 of the movable electrode 105. Thebarrier 115 isolates a first side 120 and a second side 125 of the MEMSmicrophone 100.

In some implementations, the movable electrode 105 is kept at areference voltage and a bias voltage is applied to the stationaryelectrode 110 to generate an electric sense field 130 between themovable electrode 105 and the stationary electrode 110. The electricsense field 130 is illustrated in FIG. 1 as a plurality of verticaldashes. In other implementations, the stationary electrode 110 is keptat the reference voltage and the bias voltage is applied to the movableelectrode 105 to generate the electric sense field 130. In someimplementations, the reference voltage is a ground reference voltage(i.e., approximately 0 Volts). In other implementations, the referencevoltage is a non-zero voltage.

Acoustic and ambient pressures acting on the first side 107 and thesecond side 108 of the movable electrode 105 cause deflection of themovable electrode 105 in the directions of arrows 135 and 140. Thedeflection of the movable electrode 105 modulates the electric sensefield 130 between the movable electrode 105 and the stationary electrode110. A capacitance between the movable electrode 105 and the stationaryelectrode 110 varies based on the electric sense field 130.

A phase-locked loop 200 is a control system that generates an outputsignal whose phase is related to the phase of the input signal. In someembodiments, a phase-locked loop 200 includes, among other components, avoltage-controlled oscillator 205, a reference clock 210, and a phasedetector 215, as illustrated in FIG. 2.

The voltage-controlled oscillator 205 receives a control signal andgenerates an oscillating signal based on the control signal. In someimplementations, the control signal is a voltage signal. The oscillatingsignal is a voltage signal that oscillates between to two differentvoltage levels. The frequency of the oscillating signal correlates inpart to the control signal. In some implementations, thevoltage-controlled oscillator 205 includes, among other components, anelectronic oscillator.

The reference clock 210 generates a reference signal. The referencesignal is a voltage signal that alternates (i.e., oscillates) betweentwo different voltage levels. In some implementations, the frequency ofthe reference signal is substantially constant. In some implementations,the reference clock 210 includes, among other components an electronicoscillator.

The phase detector 215 receives the oscillating signal and the referencesignal and generates the control signal. The phase detector 215generates the control signal based on a comparison of the oscillatingsignal and the reference signal. In some implementations, the phasedetector 215 generates the control signal by comparing the phase of theoscillating signal and the phase of the reference signal. The phase ofthe oscillating signal correlates to the frequency of the oscillatingsignal. Likewise, the phase of the reference signal correlates to thefrequency of the reference signal. The phase detector 215 adjusts thecontrol signal to align the phase of oscillating signal in sync with thephase of the reference signal. The control signal is substantiallyconstant when the phases match. The control signal shifts up and downbased on whether the phase of the oscillating signal is leading orlagging the phase of the reference signal. In some implementations, thefrequency of the reference signal is substantially constant and thecontrol signal directly correlates to the frequency of the oscillatingsignal.

In some implementations, the phase detector 215 includes, among othercomponents, a phase frequency detector 300, a charge pump 305, and alow-pass filter 310, as illustrated in FIG. 3. The phase frequencydetector 300 receives the oscillating signal and the reference signal.The phase frequency detector 300 compares the phase of the oscillatingsignal and the phase of the reference signal. The phase frequencydetector 300 generates an up signal and a down signal (e.g., high andlow voltage signals) based on whether the phase of the oscillatingsignal is lagging or the leading the phase of the reference signal(i.e., the phase difference). The charge pump 305 converts the up signaland the down signal into a single unfiltered signal. In someimplementations, the charge pump 305 includes, among other components, aDC-to-DC converter that uses capacitors as energy storage elements tocreate either a higher or lower voltage power source. The low-passfilter 310 converts the unfiltered signal into the control signal.

In some implementations, the voltage-controlled oscillator 205 includes,among other components, a ring oscillator 400. A ring oscillator 400includes, among other components, a plurality of inverters 405A-405C anda biasing circuit 410, as illustrated in FIG. 4. The plurality ofinverters 405A-405C is configured in a series circuit configuration. Thebiasing circuit 410 receives the control signal and generates a biasingsignal that is based on the control signal. The biasing circuit 410provides the biasing voltage to each of the plurality of inverters405A-405C. Each inverter in the plurality of inverters 405A-405C iscoupled to a reference node 415 (e.g., ground).

In order for the oscillating signal to oscillate, the ring oscillator400 requires an odd number of inverters. For example, the ringoscillator 400 illustrated in FIG. 4 includes three inverters. Eachinverter contributes an amount of propagation delay to the chain ofinverters. Propagation delay is the length of time required to change anoutput of a system in response to a change in an input of the system.The frequency of the oscillating signal correlates in part to the sum ofthe propagation delays of each of the plurality of inverters 405A-405C.Table #1 illustrates exemplary propagation delays for the ringoscillator 400 caused by a plurality of different inverter propagationdelays. The values of propagation delay in table #1 are in microseconds(μS).

The frequency of the oscillating signal is affected by the controlsignal and the propagation delay of the ring oscillator 400. Each of theplurality of inverters 405A-405C include capacitive elements (e.g., loadcapacitors) having a load capacitance that affect its propagation delay.Changing the load capacitance of at least one of the plurality ofinverters 405A-405C in the ring oscillator 400 alters the frequency ofthe oscillating signal.

In some implementations, each of the plurality of inverters 405A-405C isa complementary metal-oxide semiconductor (CMOS) inverter 500. A CMOSinverter 500 includes, among other components, a P-type metal-oxidesemiconductor (PMOS) transistor 505, an N-type metal-oxide semiconductor(NMOS) transistor 510, a load capacitor 515, an input node 520, anoutput node 525, a high voltage node 530, and a low voltage node 535, asillustrated in FIG. 5. The PMOS transistor 505 includes a gate node 540,a drain node 545, and a source node 550. The NMOS transistor 510includes a gate node 555, a drain node 560, and a source node 565. Thegate node 540 of the PMOS transistor 505 and the gate node 555 of theNMOS transistor 510 are coupled to the input node 520. The drain node545 of the PMOS transistor 505 and the drain node 560 of the NMOStransistor 510 are coupled to the output node 525. The source node 550of the PMOS transistor 505 is coupled to the high voltage node 530. Thesource node 565 of the NMOS transistor 510 is coupled to the low voltagenode 535. The load capacitor 515 is coupled between output node 525 andthe low voltage node 535.

The CMOS inverter 500 receives an input signal (e.g., input voltagesignal) at the input node 520 and generates an output signal (e.g.,output voltage signal) at the output node 525. The input signal andoutput signal alternate between a high voltage value and a low voltagevalue. The output signal is at the high voltage value when the inputsignal is at the low voltage and the output signal is at the low voltagevalue when the input signal is at the high input value. The high voltagevalue is regulated by the biasing signal and the low voltage value isregulated by the voltage of the reference node 415 (e.g., approximately0 volts). The propagation delay of the CMOS inverter 500 is the lengthof time required to change the output signal in response to a change inthe input signal. The propagation delay of the CMOS inverter 500correlates to the time required to charge and discharge the loadcapacitor 515 between the low voltage value and the high voltage value.

In some implementations, a microphone system 600 includes, among othercomponents, the MEMS microphone 100, the phase-locked loop 200, and acontroller 605, as illustrated in FIG. 6. The movable electrode 105 andthe stationary electrode 110 are coupled to the voltage-controlledoscillator 205 such that the capacitance between them acts as a loadcapacitance for the voltage-controlled oscillator 205. As explainedabove, the capacitance between the movable electrode 105 and thestationary electrode 110 changes based on acoustic and ambient pressuresacting on the movable electrode 105. Also, as explained above, thecontrol signal correlates to the frequency of the oscillating signal. Asexplained below in further detail, the frequency of the oscillatingsignal correlates in part to the capacitance between the movableelectrode 105 and the stationary electrode 110. Therefore, the controlsignal changes based on the acoustic and ambient pressures acting on themovable electrode 105.

The controller 605 includes combinations of software and hardware thatare operable to, among other things, determine an audio signal based onthe capacitance between the movable electrode 105 and the stationaryelectrode 110. In one implementation, the controller 605 includes aprinted circuit board (PCB) that is populated with a plurality ofelectrical and electronic components that provide, power, operationalcontrol, and protection to the microphone system 600. In someimplementations, the PCB includes, for example, a processing unit 610(e.g., a microprocessor, a microcontroller, or another suitableprogrammable device), a memory 615, and a bus. The bus connects variouscomponents of the PCB including the memory 615 to the processing unit610. The memory 615 includes, for example, a read-only memory (ROM), arandom access memory (RAM), an electrically erasable programmableread-only memory (EEPROM), a flash memory, a hard disk, or anothersuitable magnetic, optical, physical, or electronic memory device. Theprocessing unit 610 is connected to the memory 615 and executes softwarethat is capable of being stored in the RAM (e.g., during execution), theROM (e.g., on a generally permanent basis), or another non-transitorycomputer readable medium such as another memory or a disc. Additionallyor alternatively, the memory 615 is included in the processing unit 610.The controller 605 also includes an input/output (I/O) unit 620 thatincludes routines for transferring information and electric signalsbetween components within the controller 605 and other components of themicrophone system 600 or components external to the microphone system600.

Software included in some implementations of the microphone system 600is stored in the memory 615 of the controller 605. The softwareincludes, for example, firmware, one or more applications, program data,one or more program modules, and other executable instructions. Thecontroller 605 is configured to retrieve from memory 615 and execute,among other components, instructions related to the control processesand methods described herein. In some implementations, the controller605 includes additional, fewer, or different components.

The PCB also includes, among other components, a plurality of additionalpassive and active components such as resistors, capacitors, inductors,integrated circuits, and amplifiers. These components are arranged andconnected to provide a plurality of electrical functions to the PCBincluding, among other things, filtering, signal conditioning, orvoltage regulation. For descriptive purposes, the PCB and the electricalcomponents populated on the PCB are collectively referred to as thecontroller 605.

The controller 605 is coupled to the phase-locked loop 200 and receivesthe control signal. The controller 605 determines an audio signal basedon the control signal. As explained above, the control signal changesbased on acoustic and ambient pressures acting on the movable electrode105. Therefore, the controller 605 uses the control signal to determinethe audio signal based on acoustic and ambient pressures acting on themovable electrode 105. In some implementations, the controller 605measures a parameter (e.g., voltage, current) of the control signal anddetermines the audio signal based on the measured parameter.

FIG. 7 illustrates an exemplary configuration of the MEMS microphone 100and a CMOS inverter 500 in the voltage-controlled oscillator 205, inaccordance with some implementations of the invention. The stationaryelectrode 110 is coupled to the output node 525 of the CMOS inverter 500and the movable electrode 105 is coupled to the reference node 415. TheMEMS microphone 100 provides a load capacitance to the CMOS inverter500. The load capacitance of the CMOS inverter 500 changes as a functionof acoustic and ambient pressures acting on the movable electrode 105.This in turn causes the propagation delay of the CMOS inverter 500 tochange. Changing the propagation delay of the CMOS inverter 500modulates the frequency of the oscillating signal. Therefore, thefrequency of the oscillating signal correlates in part to thecapacitance between the movable electrode 105 and the stationaryelectrode 110.

FIG. 8 illustrates an exemplary configuration of the MEMS microphone 100and a CMOS inverter 500 in the voltage-controlled oscillator 205, inaccordance with some implementations of the invention. The movableelectrode 105 is coupled to the output node 525 of the CMOS inverter 500and the stationary electrode 110 is coupled to the reference node 415.

Coupling the movable electrode 105 and the stationary electrode 110 tothe CMOS inverter 500 does not require a high voltage pump and lowersthe impedance requirements on oxide in the MEMS microphone 100. Thisallows for a lower pull-in voltage requirement on MEMS microphone 100,greater process flexibility, and lower power consumption by themicrophone system 600.

Thus, the invention provides, among other things, systems and methods ofsensing audio with a phase-locked loop 200 whose frequency is modulatedby a MEMS microphone 100. Various features and advantages of theinvention are set forth in the following claims.

What is claimed is:
 1. A microphone system comprising: a MEMS microphoneincluding a movable electrode having a first side and a second sideopposite the first side, the movable electrode configured such thatacoustic pressures acting on the first side and the second side of themovable electrode cause movement of the movable electrode, and astationary electrode positioned on the first side of the movableelectrode; a phase-locked loop including a voltage-controlled oscillatorcoupled to the MEMS microphone that receives a control signal, andgenerates an oscillating signal based on the control signal and acapacitance between the movable electrode and the stationary electrode,and a phase detector coupled to the voltage-controlled oscillator thatreceives the oscillating signal and a reference signal, detects a phasedifference between the oscillating signal and the reference signal, andgenerates the control signal based on the phase difference; and acontroller coupled to the phase-locked loop, the controller configuredto receive the control signal, and determine an audio signal based onthe control signal.
 2. The microphone system according to claim 1,wherein the oscillating signal has a frequency based on the controlsignal and the capacitance.
 3. The microphone system according to claim1, wherein the voltage-controlled oscillator has a propagation delaybased on the control signal and the capacitance, wherein the oscillatingsignal has a phase based on the propagation delay.
 4. The microphonesystem according to claim 1, wherein the voltage-controlled oscillatorhas a ring oscillator.
 5. The microphone system according to claim 4,wherein the ring oscillator has an inverter with an input node and anoutput node.
 6. The microphone system according to claim 5, wherein thestationary electrode coupled to the output node of the inverter.
 7. Themicrophone system according to claim 6, wherein the movable electrodecoupled to a reference node.
 8. The microphone system according to claim5, wherein the movable electrode coupled to the output node of theinverter.
 9. The microphone system according to claim 8, wherein thestationary electrode coupled to a reference node.
 10. The microphonesystem according to claim 1, wherein the phase-locked loop furtherincluded a reference clock that generates the reference signal.
 11. Themicrophone system according to claim 1, wherein the phase detector has aphase frequency detector that generates an up signal and a down signalbased on the phase difference, a charge pump that generates anunfiltered signal based on the up signal and the down signal, and alow-pass filter that generates the control signal based on theunfiltered signal.
 12. A method of sensing audio with a MEMS microphone,the MEMS microphone including a movable electrode having a first sideand a second side opposite the first side, the movable electrodeconfigured such that acoustic pressures acting on the first side and thesecond side of the movable electrode cause movement of the movableelectrode and a stationary electrode positioned on the first side of themovable electrode, the method comprising: receiving, with avoltage-controlled oscillator, a control signal; generating, with thevoltage-controlled oscillator, an oscillating signal based on thecontrol signal and a capacitance between the movable electrode and thestationary electrode; receiving, with a phase detector, the oscillatingsignal and a reference signal; detecting, with the phase detector, aphase difference between the oscillating signal and the referencesignal; generating, with the phase detector, the control signal based onthe phase difference; receiving, with a controller, the control signal;and determining, with the controller, an audio signal based on thecontrol signal.
 13. The method according to claim 12, wherein theoscillating signal including a phase based on the control signal and thecapacitance.
 14. The method according to claim 12, wherein theoscillating signal includes a phase based on a propagation delay of thevoltage-controlled oscillator, wherein the propagation delay is based onthe control signal and the capacitance.
 15. The method according toclaim 12, wherein the voltage-controlled oscillator generates theoscillating signal using a ring oscillator.
 16. The method according toclaim 12, further comprising generating, with a reference clock, thereference signal.
 17. The method according to claim 12, furthercomprising generating, with a biasing circuit, a biasing signal based onthe control signal.
 18. The method according to claim 17, wherein theoscillating signal generated based on the biasing signal.
 19. The methodaccording to claim 12, wherein generating the control signal based onthe phase difference includes generating, with a phase frequencydetector, an up signal and a down signal based on the phase difference,generating, with a charge pump, an unfiltered signal based on the upsignal and the down signal, and generating, with a low-pass filter, thecontrol signal based on the unfiltered signal.